The invention relates to a process for correcting wavefront deformations which are caused by an optical system and which are measured with the aid of a wave-front-measuring instrument.
An optical system is to be understood both as an arrangement of optical components to form a functional unit, and as an individual optical component.
In accordance with its particular function, each optical system effects a specific variation in incident wavefronts. For example, plane wavefronts are converted into spherical wavefronts by an imaging optical system. However, there are deviations from the ideal shape of the wavefront upon passage through a real optical system. Wavefront deformations are produced whose causes are grounded chiefly in the non-ideal characteristics of the surfaces of the individual optical components which form the optical system. These errors in the surfaces are denoted as surface form errors or simply as form errors. Material inhomogeneities and the cementing of the optical components also play a role in causes form errors.
The errors caused in the wavefronts are composed of different error components. In particular the surface roughnesses unavoidably produced on optical components during polishing processes cause disturbed wavefronts whose disturbance or deformation is classified as 3/A (B) in accordance with DIN Standard Sheet 3140, Part 5, October 1978, pages 3-4, code number for permissible form errors. In this case, A is a measure of the spherical form error, that is to say the deviation of the surface shape from a test glass, specified by the number of Newton rings, where B describes the oval form error, that is to say the astigmatic component, likewise in the same units. The remainder of the wavefront deformation is mostly negligible in the case of the production process at issue.
The wavefront deformations can be measured with wavefront-measuring instruments such as are described, for example, in DE 40 03 698 C2 or in DE 40 03 699 C2. If the wavefront deformations measured at individual optical components or at a group of composite optical components exceed a prescribed tolerance limit, these components are taken out of the further production process and returned for reworking to a preceding process step. The reworking is performed generally only at very specific points of the optical component, which have been determined from measurement of the wavefront deformations. During the reworking, an appropriate correction is polished into the optical component. The correction acts on the wavefront as a counterdisturbance, as a result of which the original wavefront deformation is corrected up to a certain degree.
However, difficulties arise when there are high requirements for correcting the wavefront deformations. If there is a requirement for deviations from the desired ideal shape of the wavefront, where the deiration which are smaller than the wavelengths of visible light, there is an increase in the number of the optical components which exceed this tolerance limit and which therefore have to be removed from the production cycle. If, however, the rate of rejection is too high, it is no longer sensible to return the rejected optical components to a preceding process step for the purpose of correction. Firstly, frequent intervention in the fabrication cycle is very expensive in terms of time and cost and hampers fluid production. Secondly, there is a rise in the risk that a subsequent polishing process will produce additional errors. In some circumstances, more new errors will be produced than there were original errors present. Since the original errors can no longer be satisfactorily limited, limits of mastering the process have been reached. In order to achieve substantial improvements, a basically different production method would have to be developed at great expense.